Sintering Microstructure Control And Superconducting Properties Improvement In The Mg¹¹B₂ Low Activation Superconductor

MgB₂ shows excellent properties such as simple crystal structure,convenient preparation process,relatively high critical current density（Jc）and short radioactivity decay time Broad application prospect is expected for MgB₂ to be used in the international thermonuclear experimental reactor.Generally,the MgB₂ wires of various preparation methods are synthesized by using natural elementary boron as the raw material.Naturally,boron is composed of 20%¹⁰B and 80%¹¹B.In an irradiation environment,the reaction ¹⁰B+n→⁷Li+He（gas）will occur due to the large thermal neutron capture cross section of ¹⁰B.As a result,the long-term stability of the MgB₂superconducting system deteriorates.Concerning this issue,by replacing ¹⁰B with isotope ¹¹B,Mg¹¹B₂ superconducting magnets will be much more stable in neutron irradiation environments.The previous experimental studies of the Mg¹¹B₂superconducting wires showed obvious differences in contrast with the traditional MgB₂ wire.However,J_c was always not satisfying for needs of practical applications.Based on these backgrounds,in present study,we focused on the synthesis of Mg¹¹B₂,the improvement of J_c,and the clarification of the kinetics mechanism of phase formation and crystal growth mechanism of Mg¹¹B₂.Subsequently sintering technology was optimized and microstructures were effectively controlled by varying sintering process,and the current carrying capability of Mg¹¹B₂ superconductors was dramatically improved.The main results are shown as follows:Combined with X-ray diffraction and thermal analysis,the sintering process of Mg-¹¹B system was investigated.Compared with the traditional MgB₂ sample,both of the two systems were experiencing in the order of:Mg-B solid-solid reaction,Mg melting,and Mg-B liquid-solid reaction.However,the onset temperature of solid-solid reaction of Mg-¹¹B system was delayed.The results illustrated that the sintering activity of isotope ¹¹B precursor was lower than natural B precursor,hence the Mg-¹¹B system is different to the traditional Mg-B system so that they cannot be treated as equal.The effect of morphology with Mg powder on phase formation and critical current density（J_c）of sintered Mg¹¹B₂ bulk was studied.By comparing different Mg¹¹B₂samples with different morphology of Mg powder,it was found that the solid–solid reaction of Mg¹¹B₂ sample using spherical Mg powder took place ahead of schedule,but the successive peak positions of melting of Mg and the liquid–solid reaction were shifted towards higher temperatures and the intensities were increased gradually.It was primarily due to the special microstructure of spherical Mg powder that small Mg particles absorb on the surface of large Mg particles.These fine particles are so active that make it possible for Mg can react with B at a quite low temperature.Due to the limited amount of small Mg particles,the intensity of the solid–solid reaction was much weaker.Once Mg is melted,a large amount of large spherical Mg particles turn out to be highly reactive.As a result,the intensity of liquid–solid reaction gradually became stronger and shifted towards a higher temperature.On the other hand,the residual plate-like Mg powder after a solid–solid reaction will continue to participate in the liquid phase reaction until it is exhausted,so the peak intensity of plate-like Mg was a little weaker.Compared to Mg¹¹B₂ using plate-like Mg powder,the J_c values of the Mg¹¹B₂sample using spherical Mg powder are significantly improved throughout the entire magnetic field,which is primarily attributed to its unique sintered microstructure and the high purity of Mg¹¹B₂ phase.The J_c of Mg¹¹B₂ sample can be improved significantly by choosing appropriate precursor powder,but it still cannot reach the expectation of the actual requirements for applications.In order to further optimize the J_c in Mg¹¹B₂ isotope superconductors,carbon-based chemical dopants which is effective in improving the J_c of normal MgB₂with natural boron was used in this work.Compared to the undoped Mg¹¹B₂ sample,it is found that the J_c values of all doped samples within the whole measured fields were dramatically decreased instead of being improved.From the X-ray diffraction results,there was no substitution of C in the B site of the Mg¹¹B₂ lattice.To investigate the bonding characteristic of C element in the samples,we carried out high-resolution XPS analyses,which revealed that C element was bonded in various forms and obvious difference in the C 1s spectrum between doped isotope Mg¹¹B₂ samples and doped normal MgB₂ was observed.It was reported that the chemical activity of the ¹⁰B isotope is higher in comparison with ¹¹B.Thus,the substitution of C for B can only occur in the¹⁰B site of normal MgB₂,but much harder in the ¹¹B site of isotope Mg¹¹B₂.As a result,C cannot substitute B in Mg¹¹B₂ lattice,in contrast,these carbon-based additions only exist as impurities,which degrade the grain connectivity of the MgB₂ phase and thus have a negative effect on the J_c performance.Because of the ineffectiveness of Carbon-based chemical dopants in Mg-¹¹B system,in a subsequent study,minor Cu doping was chosen to further improve the current-carrying capacity of Mg-¹¹B system.From the DTA curve of the Cu-doped Mg¹¹B₂ sample,the initial temperatures of all peaks shifted towards lower figuresThe solid-solid reaction was found occur in advance by Cu addition,and minor Cu addition can significantly promote the growth of Mg¹¹B₂ grains.The whole variation tendency of MgO content in the Cu-doped samples sintered at high temperature is supposed to be higher,but another anomaly appeared for the Cu-doped sample sintered at 800 ^oC that the MgO content reduced.The temperature for the peritectic reaction Mg-Cu（l）+MgC₂（）→2)₂（）is around 800 ^oC,and the reaction M2)₂+→2)₂+2)₂ would subsequently occur.As a result,the reactions work as a loop during the holding process.The generated MgCu₂ accelerated the consumption of the Mg-Cu liquid in return,which hindered the oxidation of the Mg-Cu liquid as well.The Cu-doped samples exhibited higher J_c than the un-doped ones which were sintered at the same temperature,owing to the larger size and higher crystallinity of MgB₂ grains.The ex-situ technique currently appears to be more suitable for the development of MgB₂ superconductors,especially for long MgB₂ wires.However,the poor grain connectivity in ex-situ method is critical issue that has not been well resolved by now.In order to resolve the problem,in present work,minor Cu addition and supply of Mg were introduced into the MgB₂ samples prepared by ex-situ sintering method with the aim of improving grain connectivity.It was found that the intergranular connection among MgB₂ grains were improved and J_c was significantly enhanced.Due to the reversible reaction Mg₂?2)₄+2),MgB₂ produces more Mg that can then react with Cu,forming local Mg-Cu liquid at high temperature,which is able to enhance the self-sintering of MgB₂.However,minor Cu addition enhanced the formation of MgB₄,as well.Moderate amounts of MgB₄ can provide pinning centers,but the excess amount will limit the improvement of the intergranular connection among MgB₂ grains.For further improvement of the intergranular connection among MgB₂ grains in ex-situ method.Based on the Cu addition,the sintering processes were adjusted and controlled through prolonging the time of high temperature and extra supply of Mg powder.Prolonging the sintering time enhanced self-sintering of MgB₂,and extra supply of Mg powder can provide more liquid phase which can promote the sintering and inhibit the formation of MgB₄.After all,the intergranular connection of samples had reached a level of in-situ sample and J_c had improved remarkably.